Denial of service attacks are attempts to prevent legitimate users from utilizing or gaining access to computing resources, such as network bandwidth, memory, and CPU bandwidth. Thus, denial of service attacks make a computer resource unavailable to its intended users. Although any shared computer resource could potentially be at risk, typical targets of such attacks include high-profile web servers or other networked servers.
A common method of attack is to saturate a victim machine with external communication requests (e.g., in quantity and time) sufficient to prevent the victim machine from responding to legitimate network traffic. At the very least, such an attack can slow the response time of the victim machine to legitimate traffic. General symptoms of a denial of service attack include unusually slow network performance, unavailability of a web site, a dramatic spike in the number of spam emails received, or inability to access a network device.
One particular type of denial of service attack is a distributed denial of service (or DDoS) attack. In a DDoS attack, multiple compromised systems, also known as hosts or zombies, flood the bandwidth or resources of a targeted system. Generally, the target of a DDoS attack is one or more web servers. Essentially, the greatest point of difference between a denial of service attack and a DDoS attack is the scale. A single perpetrator acting with a single host mounts a denial of service attack, whereas a single attacker utilizing hundreds or thousands of host or zombie systems can simultaneously mount individual denial of service attacks which together amount to a DDoS attack.